The present invention relates to improved protective apparatus and methods of operation to monitor or control the operation of a power circuit breaker in electrical power transmission and distribution systems. In particular, the present invention relates to protective apparatus for checking the phase difference on line and bus sides of a closed circuit breaker, phase window adjusting apparatus, protective apparatus for checking for a discrepancy between the voltage magnitudes on the individual conductors of the line and bus when the breaker is closed, and methods of operation of the foregoing apparatus.
Three-phase alternating current (AC) systems are widely used for transmission and distribution of electric power. Faults on overhead electric power lines are generally cleared by momentarily de-energizing the line. The momentary power cutoff is accomplished by a three-phase circuit breaker, which is opened or tripped by an overload or other fault-detecting relay when an overload due to a fault is sensed. The condition of the breaker as open or closed is indicated by auxiliary contacts on the breaker conventionally designated in the industry as "52a" and "52b" contacts. The 52a contact is closed only when the breaker is closed, and the 52b contact is closed only when the breaker is open.
Automatically reclosing the breaker, after the fault clears, provides improved system stability and electric service continuity for the consumer. This, in turn, allows higher line loading by decreasing the likelihood of line loss. A microprocessor-based system for accomplishing automatic reclosing relay functions is described in detail in the coassigned application Ser. No. 881,181 cross-referenced hereinabove.
If conductors on both the line and bus sides of the breaker are energized at the time when the breaker is to be closed, they should be in approximate electrical synchronism and voltage match to avoid system shock. This is because the electrical quantities of voltage and current on the line and bus are alternating waveforms that have a phase difference that is between zero (synchronism) and .+-.180 degrees. Reclosing a circuit breaker when the line and bus are substantially out of synchronism or voltage match presents a condition resembling a temporary short circuit to the system. To avoid this condition, it has been known to use a synchronism check relay as an element in a reclosing system which senses that the voltages on the two sides of a breaker are in exact synchronism, see Applied Protective Relaying, Westinghouse 1982, chapter 20, e.g. page 20-2. Basler Electric Company of Highland, Ill., manufactures a synchronism check (sync-check) relay model BEl-25 that monitors for synchronism and voltage match before the breaker is allowed to close. Synchronism is determined when the phase difference across the open breaker is less than a manually-set amount called a phase window. A voltage monitor option is provided in that relay for determining a live or dead condition of the line and bus.
When a three-phase circuit breaker is closed, three conductive members therein connect first and second electrical conductors for each of three conductors A, B and C of the line and bus respectively. The circuit breaker may malfunction or connections thereto may fail over time, however, so that one or more of the electrical connections are not in fact made by the breaker when the auxiliary contacts indicate that it is closed. It would be desirable to provide economical and alternative ways of monitoring the circuit breaker for malfunction that are more compatible with reclosing and sync-check circuits than prior art breaker pole disagreement protection using current transformers described in Applied Protective Relaying, Chapter 16, pages 26-27.
The synchronism check relay is often located in an unattended location or used when manual adjustment of the phase window is not practical during various operating contingencies when a change of the phase window would be useful. There also is presently a need for improved apparatus and methods for effectively adjusting the phase window more conveniently.